As to the tools of life around, besides the improvement of their basic functions, people often pursue other values added, such as beauty, comfort in use, and the like. For example, as to keyboards, besides the basic typing application, since people sometimes use the computer systems in environments lacking sufficient light, luminous keyboards are produced. Corresponding to the luminous function added to the keyboards, requirements of further improvements on light uniformity, increase of illumination diversity, thinning of the keyboards, and the like are generated.